* extensions are collections of web pages that use HTML, JavaScript, CSS, so they're not difficult to write* you don't need to restart the browser after installing an extension* the extensions are updated automatically, so you'll always have the most recent version* Chrome's extensions will work in any future version of the browser. Developers don't need to update their extensions when a new version of Chrome is released.* Chrome will run extensions in separate processes: one process for each extension.

You can already write some simple extensions that add new options to the browser, but the interface is pretty limited and the APIs don't expose all the features that are available in Chrome.